/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root)
    {
        vector<int> res;
        if (root == nullptr)
        {
            return res;
        }

        TreeNode* cur = root;
        stack<TreeNode* > st;

        while (!st.empty() || cur != nullptr)
        {
            while (cur != nullptr)
            {
                st.push(cur);
                cur = cur->left;
            }
            cur = st.top();
            res.push_back(cur->val);
            cur = cur->right;
            st.pop();
        }
        return res;

    }
};